1. Field of the Invention
The present invention relates to insulated prestressed conduit strings for conducting heated fluids, and more particularly, but not by way of limitation, to an improved such conduit string formed of concentric double tube conduit sections formed together by connector means which result in high strength insulated joints.
2. Description of the Prior Art
Insulated prestressed conduit strings for conducting heated fluids through well bores comprised of a plurality of concentric double tube conduit sections and methods of constructing and assembling such conduit sections have been developed and used heretofore. For example, U.S. Pat. No. 3,511,282 to G. P. Willhite et al. discloses a conduit assembly for conveying heated fluids comprised of a plurality of concentric double tube conduit sections wherein the inner fluid conductor tube is in tension prestress and the outer tube is in compression prestress. The double tube construction of the conduit sections reduces heat loss from heated fluids conducted through the conduit assembly and the prestress condition of the tubes prevents or eliminates expansion and deformation of the assembly when heated.
U.S. Pat. No. 3,654,691 to G. P. Whillhite et al. teaches a process for constructing conduit sections of the type described above. U. S. Pat. No. 3,608,640 teaches a method of assembling a prestressed conduit in a well.
A problem encountered in the use of the insulated prestressed conduit assemblies used heretofore is that the construction of the double tube conduit sections has been such that a reduction in mechanical strength of the sections is produced at the joined ends of the sections which brings about serious limitations in the ability of the sections to be repeatedly installed in and removed from well bores. More specifically, in one design, the inner small diameter conductor tubes are connected at the joints by couplings and the outer tubes of the sections are left unconnected. This leaves a length of smaller diameter tubing between each section which is of reduced mechanical strength as compared to the double wall portions of the sections and which is uninsulated. The uninsulated portions of the conduit string allow a high rate of heat transfer from heated fluids flowing within the string thereby partially defeating the purpose of using an insulated conduit string in the well bore. Further, as indicated above, the connected-together small diameter portions of the conduit string have reduced mechanical strength as compared to the double-walled portions of the string.
In another design, the outer larger diameter tubes of each conduit section are connected together by couplings thereby eliminating the high heat transfer problem at the joints mentioned above, but the portions of the outer tubes of the conduit sections which are connected together at the joints are attached to bushings which are in turn attached to the inner tubes of the sections at a distance from the joints whereby the strength of the joints is limited to the strength of the outer tubes only.
By the present invention an improved insulated prestressed conduit string for conducting heated fluids through a well bore is provided which obviates the problems mentioned above and has a greater resistance to wear and tear during reuse of the conduit string.